The present exemplary embodiment relates to a method for forming a compound semiconductor thin-film. It finds particular application in conjunction with semiconductor thin-films suitable for use in photovoltaic solar cells and other devices, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Photovoltaic devices represent one of the major sources of environmentally clean and renewable energy. They are frequently used to convert solar energy into electrical energy. Typically, a photovoltaic device is made of a semiconducting junction with p-type and n-type regions. The conversion efficiency of solar power into electricity of such devices is limited to a maximum of about 30%, since photon energy in excess of the semiconductor's bandgap is wasted as heat and photons with energies smaller than the bandgap do not generate electron-hole pairs. The commercialization of photovoltaic devices depends on technological advances that lead to higher efficiencies, lower cost, and stability of such devices.
The cost of electricity can be significantly reduced by using photovoltaic devices constructed from inexpensive thin-film semiconductors. Thin films of polycrystalline copper indium gallium selenide of the form Cu(In1-xGax)Se2, 0≦x≦1 (CIGS), have shown promise for applications in thin film photovoltaics. The band gaps of these materials range from approximately 1.1 to 1.7 eV (see, J. L. Shay and J. H. Wernick, “Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties and Applications,” Pergamon, N.Y. (1975)). This should allow efficient absorption of solar radiation. A solar cell with an efficiency of 19.9%, measured with AM1.5 illumination, has recently been demonstrated by Repins, et al. (I. Repins, et al., “19.9%-efficient ZnO/CdS/CuInGaSe2 solar cell with 81.2% fill factor,” Progress in Photovoltaics: Research and Appl., 16, 235-239 (2008)). See also, K. W. Mitchell, Proc. 9th E. C. Photovoltaic Solar Energy Conference, Freiburg, FRG, September 1989, p. 292. Kluwer, Dordecht (1989); M. A. Green, et al., Prog. Photovolt. Res. Appl. 15, 35 (2007); Report on the Basic Energy Sciences Workshop on Solar Energy Utilization, US Dept. of Energy, Apr. 18-21, 2005; J. D. Beach, B. E. McCandless, Mater. Res. Bull. 32, 225 (2007); and M. A. Contreras, et al., Pro. Photovolt. Res. Appl. 13, 209-216 (2005).
CIGS films have been vacuum deposited by several different methods. These include evaporation (see, Repins, et al.), two-stage processes utilizing evaporated or sputter deposited precursors followed by selenization in H2Se (see B. M. Basol, “Preparation techniques for thin film solar cell materials: processing perspectives,” Jph. J. Appl. Phys. 32, 35 (1993); E. Niemi and L. Stolt, “Characterization of CuInSe2 thin films by XPS,” Surface and Interface Analysis 15, 422-426 (1990)), metallic ink coating (G. Norsworthy, et al., “CIS film growth by metallic ink coating and selenization,” Solar Energy Materials & Solar Cells 60, 127-134 (2000)), and coating via soluble hydrazine-based precursors (D. B. Mitzi, et al., “A high-efficiency solution-deposited thin-film photovoltaic device,” Adv. Mater. 20, 3657-3662 (2008)).
While such techniques have produced efficient devices in the laboratory, there remains a need for CIGS deposition technologies that are scalable to large-area devices for commercial applications. Techniques for sputter deposition of CIGS, for example, have included the costly and potentially hazardous step of further selenization in H2Se of the previously sputtered elements. Films made by sputtering directly from the CIGS compounds are Se-poor since selenium is lost in the vapor phase during film deposition (see, V. Probst, et al., “Rapid CIS-process for high efficiency PV-modules: development towards large area processing,” Thin Solid Films, 387, 262-267 (2001)). Additionally, the morphology is very coarse. Conventional sputtered CIGS films are thus typically unsuitable for high efficiency photovoltaic devices.